Page 205 - Wind Energy Handbook

CONSTANT ROTATIONAL SPEED OPERATION 179

which will inevitably be accompanied by large bending moments and stresses,
causing fatigue damage. When parked in high, turbulent winds a rotor with fixed
pitch blades may well be subject to large aerodynamic loads which cannot be
alleviated by adjusting (feathering) the blade pitch angle. Consequently, a fixed-
pitch, stall-regulated turbine experiences more severe blade and tower loads than a
pitch regulated turbine.

4.2.3 Effect of rotational speed change

The power output of a turbine running at constant speed is strongly governed by
the chosen, operational rotational speed. If a low rotation speed is used the power
reaches a maximum at a low wind speed and consequently it is very low. To extract
energy at wind speeds higher than the stall peak the turbine must operate in a
stalled condition and so is very inefficient. Conversely, a turbine operating at a high
speed will extract a great deal of power at high wind speeds but at moderate wind
speeds it will be operating inefficiently because of the high drag losses. Figure 4.7
demonstrates the sensitivity to rotation speed of the power output – a 33 percent
increase in r.p.m. from 45 to 60 results in a 150 percent increase in peak power,
reflecting the increased wind speed at which peak power occurs at 60 r.p.m.
At low wind speeds, on the other hand there is a marked fall in power with
increasing rotational speed as shown in Figure 4.8. In fact, the higher power
available at low wind speeds if a lower rotational speed is adopted has led to two-
speed turbines being built. Operating at one fixed speed which maximizes energy
capture at wind speeds at, or above, the average level will result in a rather high
cut-in wind speed, the lowest wind speed at which generation is possible. Employ-

80

60 rpm

60 55 rpm
Electrical power (kW) 40 50 rpm

20 45 rpm

0
0 5 10 15 20 25
Wind speed (m/s)
Figure 4.7 Effect on Extracted Power of Rotational Speed